1. Field of the Invention
The present invention relates to a high-voltage generating circuit, and more particularly to a high-voltage generating circuit for use in a CRT display apparatus. More specifically, the present invention is concerned with a high-voltage generating circuit for use in a CRT display apparatus which is automatically responsive to a signal source having a wide range of horizontal frequencies.
2. Description of the Relevant Art
One general high-voltage generating circuit for use in a CRT display apparatus comprises, as shown in FIG. 1 of the accompanying drawings, a horizontal oscillator and AFC (automatic frequency control) circuit 1, a horizontal drive circuit 2, a high-voltage converter output circuit 3, and a flyback transformer 4. The horizontal oscillator and AFC circuit 1 is supplied with a horizontal synchronizing signal H.sync and also with flyback pulses FBP that are fed back from the output terminal of a horizontal deflection circuit (not shown). The horizontal oscillator and AFC circuit 1 effects an automatic frequency control process to generate horizontal frequency pulses which are horizontally synchronized.
The horizontal oscillator and AFC circuit 1 generally doubles as a drive circuit for the horizontal deflection circuit. The oscillation frequency of a high voltage as well as the frequency of horizontal deflection pulses varies depending on the horizontal frequency of an input signal applied to the horizontal oscillator and AFC circuit 1.
In a multiscan-type display apparatus, for example, the horizontal oscillator and AFC circuit 1 is arranged to respond to a wide range of horizontal frequency signals applied thereto. The horizontal frequency signals include a computer graphic signal of 64 KHz, a high-density videotext signal of 31.5 KHz, and a television signal of 15.7 KHz.
The conventional high-voltage generating circuit for use in a CRT display apparatus suffers various drawbacks when the CRT display apparatus displays an image based on a signal whose horizontal frequency is lower than a horizontal frequency f.sub.H for which the high-voltage generating circuit is originally designed, e.g., an image based on a television signal or a high-density videotext signal. Specifically, the impedance of the choke coil is lowered to allow a large direct current to flow therethrough, giving off an increased amount of heat. Since the horizontal frequency of the image signal is low, the period of time in which a high-voltage load is discharged is increased, resulting in large high-voltage fluctuations. Inasmuch as the horizontal drive transformer cannot continuously supply a sufficient drive current, the high-voltage output transistor fails to operate properly.
To eliminate the above shortcomings, it has been proposed to change inductance values of the choke coil depending on the scanning frequency, change resonant capacitors to increase the duration of flyback pulses, and change horizontal drive transformers. However, these proposals are disadvantageous in that they require a large circuit scale, are expensive as power devices are needed to change the inductance values of the choke coil, the resonant capacitors, and the horizontal drive transformers, and do not provide sufficient merits.
Another solution is to produce a high voltage based on an asynchronous oscillation irrespective of the frequency of the input signal. Since asynchronous noise would appear on the displayed image, however, a magnetic field leakage has to be shielded or high-voltage ripples have to be suppressed to remove such asynchronous noise. Therefore, this solution also poses a cost problem because of the additional measures for asynchronous noise removal.